Process for coating metals

Abstract

The invention provides a process for forming a ceramic coating on a valve metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals. The process comprises the steps of (a) immersing the metal as an electrode in an electrolytic bath comprising water and a solution of an alkali metal hydroxide; (b) providing an opposite electrode immersed in or containing the electrolyte liquid; (c) passing a modified shaped-wave alternate electric current from a high voltage source of at least 700 V through a surface of the metal to be coated and the opposite electrode, thereby causing dielectric breakdown, heating, melting, and thermal compacting of a hydroxide film formed on the surface of the metal to form and weld a ceramic coating thereto, and (d) changing the composition of the electrolyte while the ceramic coating is being formed, the change being effected by adding an oxyacid salt of an alkali metal. In a preferred embodiment, the modified shaped-wave electric current rises from zero to its maximum height and falls to below 40% of its maximum height within less than a quarter of a full alternating cycle. Also provided is apparatus for carrying out this process.

Dependencies are inferred from the claim text.